Human evolution is a sensitive and controversial topic, which might explain why it is not included in science curricula or textbooks in many countries. We prepared an online student-centered human evolution activity dealing with lactose tolerance. In constructing the activity, we considered the following design principles: a medical issue connected to students' lives, a noncontentious topic of human evolution, and a one-step genetic example that can be demonstrated by basic bioinformatics tools. The activity consists of four units dealing with the activity of the enzyme lactase in our small intestine, the differences in lactose tolerance in people from different origins, the genetic foundation of lactose tolerance, and an extension unit dealing with the control of lactase gene expression. The activity was experienced by a pilot group of approximately 100 students, preservice and in-service teachers who showed great interest in the genetics of a trait that has undergone evolutionary changes. We noted the need for a teacher as mediator while students perform the activity. We suggest using the activity in the context of evolution, genetics, or when teaching about systems of the human body, either all units in succession or as a modular activity.